This invention relates to improvements in the structure of and method of making heat insulated pistons and valves for internal combustion engines such as gasoline and diesel engines.
The efficiency of internal combustion engines, and in particular diesel engine, is improved by retaining heat from the ignited fuel in the combustion chamber of the cylinder by minimizing heat loss through the piston. This has been done in the past by insulating the piston and valve heads by various techniques described below. A problem with these techniques, however, has been high manufacturing costs and the strength and durability of the bond between the ceramic member and the metal piston or valve. To date no reliable bond between curved surfaces of the ceramic member and the metal component has been devised.
U.S. Pat. No. 4,242,948 describes a piston which uses a metal bolt to attach a ceramic cap to the piston body. The metal bolt, however, expands during operation and tends to loosen the connection of the cap to the piston. Any ceramic chip between the cap and the piston will force the cap to break when the engine cools to room temperature and the bolt shrinks.
Another proposal is to shrink fit a carefully ground ceramic liner into a steel piston cap. This construction is susceptible to failure from ceramic cracks due to the irregular shear force which exceeds the mechanical property of the ceramic material.
U.S. Pat. No. 4,245,611 describes a piston which has a ceramic insert cast into a cavity in the piston body. This technique is limited to pistons having bodies with high melting temperatures and precludes the use of cast iron pistons. In addition, differential thermal coefficients of expansion between the ceramic and metal body tend to shrink the metal to a greater degree than the ceramic and cause damage to the parts.
In general, success in insulating pistons with ceramic material has been limited because of the difficulty of attaching the ceramic piece to the metal piston body without stressing the ceramic piece during the manufacturing process or operation of the engine.